In recent years, a reduction of friction is an important function of lubricating oil compositions for the achievement of energy savings and fuel savings. The use of a lubricating oil composition having a high viscosity, however, is disadvantageous because a load is applied to a pump for feeding the lubricating oil composition and because a loss in stirring the lubricating oil composition is caused. To cope with these problems, an attempt is generally made to reduce the viscosity of a lubricating oil composition. In the case of a lubricating oil composition whose viscosity is reduced by a conventional method, however, there is a problem because a loss of the lubricating oil composition is caused due to evaporation especially when it is used in a high temperature environment. Further, there is a problem that the coefficient of friction of a lubricating film rather increases with a reduction of the viscosity of the lubricating oil composition.
On the other hand, internal combustion engines are more and more required to be of a fuel saving type. To meet with such a requirement, the viscosity of an engine oil is further reduced. However, a low viscosity oil, which is susceptible to be vaporized when subjected to high temperatures in the engine, is discharged together with a waste gas during use. Therefore, the viscosity of the low viscosity oil gradually increases to cause an increase of the fuel consumption. With a view toward solving such problems of the conventional oils, Noack test has been newly introduced as an index of vaporizability. Thus, there is an increasing utilization of a low viscosity base oil which meets with the requirement for low vaporizability.
When a mineral base oil is used for the purpose of obtaining a low viscosity lubricating oil composition, there is caused a problem that the amount of vaporization of the lubricating oil composition considerably increases. Thus, an attempt has been made to use various synthetic base oils. As a lube base oil having a low viscosity and a low vaporizability, an ester type base oil is known (see, for example, Patent Document 1 and Non-Patent Document 1). The ester type base oil, however, has a high polarity and causes an adverse effect (swelling) on organic materials. For example, when such an oil is used as an engine oil, there is caused a problem of swelling a rubber used as a seal material, etc. When such an oil is used as a fluid dynamic bearing oil, etc., there is caused a problem that a plastic material is adversely affected. There is also caused a problem that solubility of an additive in such an oil is poor.
A silicone oil is a lube base oil having a low viscosity and a low vaporizability, but has a problem because it has a poor lubricating property and is expensive. Poly-α-olefin (for example, an oligomer of 1-decene) is also known as a base oil having a low viscosity and a low vaporizability. However, poly-α-olefin of a viscosity grade of 4 mm2/s (100° C.) is not fully satisfactory with respect to a low viscosity and poly-α-olefin of a viscosity grade of 2 mm2/s (100° C.) is not fully satisfactory with respect to a low vaporizability.
Additionally, disclosed is a lube base oil containing a compound having an ether bonding and limiting to specific ranges of viscosity coefficient and pour point (see, for example, Patent Document 2). To meet with the requirement of further fuel saving, there is still a demand for a lube base oil having a lower viscosity and a lower vaporizability.
Patent Document 1: Japanese Unexamined Patent Publication Hei 08-245504
Non-Patent Document 1: “TRIBOLOGIST”, 38(1), p 28-31 (1993)
Patent Document 2: Japanese Unexamined Patent Publication Hei 10-324883